Photochemistry

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Photochemistry

Photochemistry, a sub-discipline of chemistry

Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, is the study of the interactions between atoms, small molecules, and light (or electromagnetic radiation

Electromagnetic radiation

Electromagnetic radiation takes the form of wave propagation waves in a vacuum or in matter. EM radiation has an electric field and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy Wave propagation....
). The pillars of photochemistry are UV/VIS spectroscopy, photochemical reaction

Photochemical reaction

A photochemical reaction is a chemical reaction which is induced by electromagnetic wave . Examples of photochemical organic reactions are electrocyclic reactions, photoisomerization and Norrish reactions....
s in organic chemistry and photosynthesis

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
in biochemistry.

most scientific disciplines, photochemistry utilizes the SI or metric measurement system.

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Encyclopedia

Photochemistry, a sub-discipline of chemistry

Chemistry

Electromagnetic radiation

Photochemical reaction

Photosynthesis

File:Seawifs global biosphere.jpgPhotosynthesis is a metabolic pathway that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight....
in biochemistry.

Scientific background

Like most scientific disciplines, photochemistry utilizes the SI or metric measurement system. Important units and constants that show up regularly include the meter (and variants such as centimeter, millimeter, micrometer, nanometer, etc.), seconds, hertz, joules, moles, the gas constant

Gas constant

The gas constant is a physical constant which is featured in a large number of fundamental equations in the physical sciences, such as the ideal gas law and the Nernst equation....
R, and the Boltzmann constant

Boltzmann constant

The Boltzmann constant is the physical constant relating energy at the particle level with temperature observed at the bulk level. It is the gas constant R divided by the Avogadro constant NA:...
. These units and constants are also integral to the field of physical chemistry

Physical chemistry

Physical chemistry is the application of physics to macroscopic, microscopic, atomic, subatomic, and particulate phenomena in chemical systems within the field of chemistry traditionally using the principles, practices and concepts of thermodynamics, quantum chemistry, statistical mechanics and kinetics....
.

The first law of photochemistry, known as the Grotthuss-Draper law

Grotthuss-Draper law

The Grotthuss?Draper law states that only that light which is absorbed by a system can bring about a photochemical change. It was first proposed in 1817 by Theodor Grotthuss and John W. Draper....
(for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction

Photochemical reaction

Stark-Einstein law

The Stark?Einstein law is named after the German-born physicists Johannes Stark and Albert Einstein, who independently formulated the law between 1908 and 1913....
, states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction. This is also known as the photoequivalence law and was derived by Albert Einstein

Albert Einstein

Albert Einstein was a Germany-born theoretical physics. He is best known for his theory of relativity and specifically mass?energy equivalence, expressed by the equation E = mc2....
at the time when the quantum (photon) theory

Quantum mechanics

Quantum mechanics is a set of principles underlying the most fundamental known description of all physical systems at the microscopic scale . Notable amongst these principles are both a dual wave-like and particle-like behavior of matter and radiation, and prediction of probabilities in situations where classical physics predicts certaintie...
of light was being developed.

Photochemistry may also be introduced to laymen as a reaction

Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
that proceeds with the absorption of light. Normally a reaction (not just a photochemical reaction) occurs when a molecule gains the necessary activation energy

Activation energy

In chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius, that is defined as the energy that must be overcome in order for a chemical reaction to occur....
to undergo change. A simple example can be the combustion

Combustion

Combustion or burning is a complex sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames, appearance of light flickering....
of gasoline

Gasoline

File:GasCan.jpgGasoline or petrol is a petroleum-derived liquid mixture, primarily used as fuel in internal combustion engines.It consists mostly of aliphatic hydrocarbons, enhanced with iso-octane or the aromatic hydrocarbons toluene and benzene to increase its octane rating....
(a hydrocarbon

Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "pure" hydrocarbons, whereas other hydrocarbons with bonded com...
) into carbon dioxide and water. This is a chemical reaction where one or more molecules/chemical species

Chemical species

Chemical species are atoms, molecules, molecular fragments, ions, etc., as entities being subjected to a chemical process or to a measurement. Generally, a chemical species can be defined as an ensemble of chemically identical molecular entity that can explore the same set of molecular energy levels on a characteristic or delineated time scal...
are converted into others. For this reaction to take place activation energy should be supplied. The activation energy is provided in the form of heat or a spark. In case of photochemical reactions light provides the activation energy.

The absorption of a photon of light by a reactant molecule may also permit a reaction to occur not just by bringing the molecule to the necessary activation energy, but also by changing the symmetry of the molecule's electronic configuration, enabling an otherwise inaccessible reaction path, as described by the Woodward-Hoffmann selection rules

Woodward-Hoffmann rules

The Woodward?Hoffmann rules devised by Robert Burns Woodward and Roald Hoffmann are a set of rules in organic chemistry predicting the stereochemistry of pericyclic reactions based on orbital symmetry....
. A 2+2 cycloaddition reaction is one example of a pericyclic reaction

Pericyclic reaction

In organic chemistry, a pericyclic reaction is a type of organic reaction wherein the transition state of the molecule has a cyclic geometry, and the reaction progresses in a concerted reaction fashion....
that can be analyzed using these rules or by the related frontier molecular orbital

Molecular orbital

In chemistry, a molecular orbital is a mathematical function that describes the wave-like behavior of an electron in a molecule. This function can be used to calculate chemical and physical properties such as the probability of finding an electron in any specific region....
theory.

Spectral regions

Photochemists typically work in only a few sections of the electromagnetic spectrum

Electromagnetic spectrum

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation from that particular object....
. Some of the most widely used sections, and their wavelengths, are the following:

Ultraviolet
Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 400 nanometer to 10 nm, and energies from 3 Electron volt to 124 eV....
: 100–400 nm
Visible Light: 400–700 nm
Near infrared: 700–2500 nm
Mid infrared: 2500 - 25000 nm
Far infrared: 25–1000 µm

Applications

There are important processes based in the photochemistry principles. One case is photosynthesis

Photosynthesis

Glucose

Glucose , a monosaccharide also known as grape sugar, blood sugar, or corn sugar, is a very important carbohydrate in biology....
in their chloroplasts to contribute to cell metabolism. The glucose is used by the plant's mitochondria to produce adenosine triphosphate

Adenosine triphosphate

This article is about the chemical used by cells as an energy carrier. For other uses, see ATP .Adenosine-5'-triphosphate is a multifunctional nucleotide, and plays an important role in cell biology as a coenzyme that is the "molecule unit of currency" of intracellular energy transfer....
. Medicine bottles are made with darkened glass to prevent the medicine itself from reacting chemically with light. In fireflies

Fireflies

Fireflies is a novel by Shiva Naipaul originally published in 1970. It was his first book, a comic novel set in Trinidad. In an essay in An Unfinished Journey, Naipaul described how in 1968 as a final year student at Oxford University studying Chinese, he had been moved to write down a sentence, which proved to be the beginning of his fir...
, an enzyme

Enzyme

Enzymes are biomolecules that catalysis chemical reactions. Almost all enzymes are proteins. In enzymatic reactions, the molecules at the beginning of the process are called Substrate , and the enzyme converts them into different molecules, the products....
in the abdomen works to produce bioluminescence

Bioluminescence

Bioluminescence is the production and emission of light by a living organism as the result of a chemical reaction during which chemical energy is converted to light energy....
. The mercaptans or thiols produced by Chevron Phillips

Chevron Phillips

Chevron Phillips is a chemical producer jointly owned by Chevron Corporation and ConocoPhillips. The company was formed July 1, 2000 by merging the chemicals operations of both Chevron Corporation and Phillips Petroleum Company....
Chemical Company are produced by photochemical addition of hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the chemical formula Hydrogen2Sulfur. This colorless, toxic and flammable gas is partially responsible for the foul odor of egg and flatulence....
(H₂S) to alfa olefins. Among their many uses as a chemical reagent these mercaptans are used to provide a distinctive odor (an odorant) to otherwise odorless natural gas

Natural gas

Natural gas is a gas consisting primarily of methane. It is found associated with fossil fuels, in coal beds, as methane clathrates, and is created by methanogenic organisms in marshes, bogs, and landfills....
. Many polymerizations are started by photoinitiatiors which decompose upon absorbing light to produce the necessary free radicals for Radical polymerization

Radical polymerization

Radical polymerization is a type of polymerization in which the reactive center of a polymer chain consists of a radical .The polymerization reaction is initiated by three classes of radical initiators:...
.

Photochemistry

Scientific background

Spectral regions

Applications

See also